Pressure molding of glass articles apparatus

ABSTRACT

APPARATUS FOR PRESSURE GLASS SHEETS, IN PARTICULAR HEMISPHERES, BY CASTING MOLTEN GLASS UNDER PRESSURE BETWEEN GRAHPIT MOLDS WHILE THE MOLDS ARE MAINTEINED WITHIN A DESIRED TEMPRATURE RANGE BELOW A TEMPERATURE AT WHICH MOLTEN GLASS ADHERES TO GRAPHITE AND ABOVE A TEMPERATURE AT WHICH THE MOLDS CAUSE CHILL CRACKING AND OPTICAL DEFECTE IN THE SURFACES OF THE CAST GLASS.

June 4, 1974 J. N. PARRIS 3,814,593

PRESSURE MOLDING 0F GLASS ARTICLES APPARATUS Original Filed Oct. 21.1970 2 1 WI" I FIO.1

June 4, 1974 J. N. PARRIS 3,814,593

PRESSURE MOLDING OF GLASS ARTICLES APPARATUS 2 Shoots-Shout 2 OriginalFlled Oct. 21. 1970 United States Patent Oflice 3 Patented June 4, 1974Int. Cl. C03b 11/00 US. Cl. 65-305 4 Claims ABSTRACT OF THE DISCLOSUREApparatus for pressure forming glass sheets, in particular hemispheres,by casting molten glass under pressure between graphite molds while themolds are maintained within a desired temperature range below atemperature at which molten glass adheres to graphite and above atemperature at which the molds cause chill cracking and optical defectsin the surfaces of the cast glass.

This is a division of application Ser. No. 82,768, filed Oct. 21, 1970,now US. Pat. No. 3,725,023, granted Apr. 3, 1973.

BACKGROUND OF THE INVENTION The present invention relates to apparatusfor pressure forming glass shapes from molten glass. While the presentinvention resulted from a need to improve the characteristics of glasshemispheres, the techniques taught by the present invention are alsosuitable for the formation of a variety of glass shapes, for example,structural shapes such as I-beams and the like, glass statues, and anyother desired glass shape.

One important end product made possible by the present invention is ahollow glass sphere having a nominal wall thickness of at least one halfinch and an outside diameter in excess of twelve inches. These spheresare produced by laminating a pair of hemispheres fabricated according tothe present invention along their equators. A preferred method ofjoining hemispheres together is described in U."S. Pat. No. 3,450,080 toHenry M. Demarest, Jr. Spheres so produced have found great utility ascontainers for instruments in deep submergence studies under water.

Prior to the present invention, fiat slabs of glass were formed intohemispherical shapes either by sag bending, for example, by a techniquedescribed in US. Pat. No. 3,560,183 of George W. Stilley and John A.Comperatore, for Bending Glass Sheets, or by a combination of gravitysagging and pressure bending as described in US. Pat. No. 3,414,395 toThomas J. Reese and Russell J. Corsi.

While such prior art techniques produced hemispheres of the desiredshape, it was diflicult to handle the flat slab that formed the workpiece for making the hemisphere. In addition, hemispheres so formed havemold marks on their surface that require polishing for removal anddifiiculty was encountered in controlling the uniformity of thickness ofhemispherical shapes so produced. Thus, it was often necessary to grindthe thicker parts of the hemispheres whenever uniformity of thicknesswas required.

The present invention provides apparatus for forming glass into adesired shape, preferably a hemispherical shape. The method, performedby apparatus conforming to the present invention involves establishing aspace of desired thickness between a pair of graphite molds of male andfemale configuration, respectively, that defines the desired shape,melting and refining more glass than is needed to fill said space,forcing said melted and refined glass upward at superatmosphericpressure into the space between said molds until said space iscompletely filled with said glass while maintaining said molds at aminimum temperature sufiicient to obtain momentary temperatureequilibrium at the glass-mold interfaces above the glass annealing pointand below a temperature at which the glass adheres to the mold. In apreferred embodiment of the present invention, the male mold is over thefemale mold.

The temperature of the molds is then cooled at a controlled rate tocontrol the rate of cooling the glass. When the mold temperatures arewithin a temperature range at which said glass is sufiiciently viscousto maintain its shape, yet still above its strain point, the male moldis retracted upward from the body of glass suflicient distance to permitfree thermal contraction of the cooling glass from the cooling femalemold. The female mold continues to support the glass until the glass iscooled to below its strain point.

The glass is melted within a chamber that is capable of beingpressurized with a non-oxidizing gas and preferably an inert gas such asargon. The pressure forces the molten glass to rise through a deliverytube into the established space at a relatively rapid rate.

In the past, molten metal, but not glass, has been pressure cast usingcold graphite molds, as shown in US. Pat. No. 2,839,801 to Zickefoose.The use of cold graphite molds is not suitable for pressure castingglass sheets because when the glass set on contacting the cold mold,such setting produces chill cracking and optical flaws in the glasssurface. Furthermore Zickefoose uses air to pressurize the molten glassfor pressure casting. Air oxidizes the graphite molds heated by themolten metal introduced therebetwee and spoils the shape of the moldcavity.

It has also been attempted to gravity cast molten glass in a mold at atemperature of about 2500 degrees Fahrenheit. The molten glass adheresto the mold at such high temperatures. As a result, using such a hotmold for casting glass is a slow and tedious process and requires asubsequent grinding operation to remove the mold portion that adheres tothe glass surface. The hot mold cannot be reused unless reworked.

The present invention provides upward pressure casting rather thangravity feed to molten glass and avoids the need of grinding to removean adhered mold from the surface of a cast glass article by controllingthe temperature of the graphite mold during pressure casting so that themold-glass interface attains a desired temperature range above theannealing point of the glass, and below the temperature at whichgraphite would react chemically with or adhere to the glass.Furthermore, the present invention uses a non-oxidizing or an inertatmosphere to further avoid chemical reaction between the molten glassand the mold or oxidation of the mold.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be betterunderstood in the light of the description of a specific embodimentthereof.

In the drawings forming part of the description and wherein likereference numbers refer to like structural elements, FIG. 1 is anelevational view, partly in section, of an illustrative embodiment ofapparatus for performing the present invention for producinghemispheres;

FIG. 2 is a plan view of the apparatus illustrated in FIG. 1; and

FIGS. 3 and 4 are enlarged fragmentary sectional views of a valveforming part of the illustrative embodiment, shown in its open andclosed position, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENT The apparatus comprises acylindrical steel pressure shell 11 surrounded by water cooled pipes 12of copper brazed to the outer wall of the shell by a silver solder. Itis understood, however, that the pressure shell may be of other shapethan cylindrical, such as a rectangular cross-section. Additionalcooling pipes 14 are coiled below the floor 16 of the pressure shell 11.A furnace 18 of refractory material is supported within the pressureshell 11 by suitable insulation such as a graphite felt.

A pair of graphite bus bars 21 and 22 connected to a voltage source (notshown) extend through the wall of the pressure shell and continuethrough the insulation of a wall of the furnace 18. The bus bars areconnected electrically to one another through a helical graphite heatingelement 24 supported on the wall of the furnace 18. While graphite isspecified for the bus bars and heating elements, other high temperatureelectroconductive materials, such as tungsten, molybdenum and the like,may also be used. A window 26 is provided to present an obliquelydownward line of sight through the wall of the pressure shell 11 andinto the furnace 18. A steel cover 27 is attached to the pressure shell11 to enclose the pressure shell and the furnace.

Centrally disposed within the furnace is a container 28. A delivery pipe30 is concentrically mounted to extend from a short distance above thebottom of the container 28 vertically upward through an opening 31 inthe cover 27 where the pipe 30 is supported by a packing gland 32. Theupper end of the pipe 30 extends through the floor of a mold housing 33of steel and terminates adjacent an adapter 34 for a ball valve 35supported in the lowest portion of a female mold 36 within the moldhousing 33. The adapter and valve are composed of graphite as is thefemale mold 36. It is understood that while a ball valve is disclosed,it is equally feasible to use a slide valve or any other suitable valveto disconnect a mass of molten glass above the female mold 36 from themolten glass in delivery tube 30.

A pressure pipe 38 is connected to a source of argon and extendstherefrom into the pressure shell 11. A pressure relief pipe 40 havingan adjustable valve extends from within the pressure shell 11 to outsidethe pressure shell 11 to relieve the pressure within the pressure shellwhen needed. The furnace 18 and the container 28 are exposed to thepressure within the pressure shell 11. Pipes 38 and 40 are valved forpressure control.

The female mold 36 has an upward facing shaping surface 42 ofhemispherical shape terminating in a peripheral shoulder 43 and issurrounded by electroconductive tubes of Inconel 44 helically woundabout the outer wall of the female mold. The Inconel tubes are connectedto lead in tubes 46, which are in turn connected to a source of voltage.In addition, water or other cooling medium can be circulated through thetubes 44 and 46 to provide a means for controlling the temperature ofthe female mold 36.

A valve stem 47 is connected to the valve 35 and rotates the latterabout the axis of the valve stem between an open position shown in FIG.3 and a closed position seen in FIG. 4. The ball valve 35 issubstantially spherical except for a concave recess 37 (FIG. 4)intermediate ends of an aperture 39 extending through the ball valve 35.The concave recess 37 provides a smooth continuation of the uppersurface 42 of the female mold 36 when valve 35 is in the closed positionof FIG. 4.

The apparatus also comprises a male mold 48 having a downward facingsurface 50 complementary to the upper surface 42 of the female mold 36.The male mold 48 is disposed above the female mold 36. The centralportion of the male mold 48 is recessed to receive Inconel tubes 52similar to the Inconel tubing 42 and 46 provided in the female mold 36.The upper portion of the male mold 48 is provided with a peripheralflange 54 superimposed over the peripheral shoulder 43 of the femalemold 36. A stainless steel ring 56 having a graphite insert 58 isdisposed between the lower surface of the flange 54 of the upper malemold 48 and the peripheral shoulder 43 of the lower female mold 36. Thegraphite insert 58 is disposed across the upper portion of the space 59existing between the downward facing surface 50 of the male mold 48 andthe upward facing surface 42 of the female mold 36 to serve as a coverand retainer for glass delivered into said space 59.

A metal cover 60 is provided above the upper mold 48. The cover isattached to the mold housing 33. The latter is concentric with andimmediately above the roof 27 of the pressure shell 11 to which it issuitably attached.

The mold covering 60 is suitably reinforced by a box like structure 66.The cover 60 and the box like structure are apertured to receive theInconel tubes 52. Three additional apertures are provided to receiveexternally threaded shafts 68 of jacks 70 operated in unison by a crank71 off a main shaft 72, auxiliary shafts 74, and bevel gearing 76 and78. The jacks operate to retract the male mold 48 upwardly from thefemale mold 36 either simultaneously with or shortly after the glassfills the hemispherical space 59. The steel ring 56 with its graphiteinsert 58 retains the glass hemisphere against the upper surface 42 ofthe female mold 36 when the jacks 70 lift the male mold 48.

The following examples are provided to indicate how the presentinvention is used to form glass hemispheres of different sizes presentlyin demand.

EXAMPLE I Hemispheres of 13 inch outer diameter and one half inch thickwere produced of the following glass composition: 61 weight percent ofSiO 15.5 weight percent of A1 0 10 weight percent of B 0 4.5 weightpercent of U 0 and 9 weight percent of MgO. This composition has anannealing point of 1055 degrees Fahrenheit. Its viscosity-temperaturecurve is approximately equal to that of commercial soda-lime-silicaplate glass. The molds used were of dense grade graphite, NationalCarbon Company grade ATJ. This graphite has a maximum grain size of .006inch diameter, and an ash content of 0.2%, a bulk density of 1.73 gramsper cubic centimeter, a flexural strength of 3300 pounds per square inchand an electrical resistivity of '11.70 10" ohm centimeters. The malespherical mold weighed 30 pounds and had a diameter of twelve inches.The female mold weighed approximately 40 pounds and had an upper surfaceof 13 inches in diameter. If less dense graphite is available, it isnecessary to use heavier molds than those used in the reportedexperiments to obtain an equally good quality of glass surface finish.

The glass composition was heated to a temperature of 2680 degreesFahrenheit. The molds were held at temperatures of 900 degreesFahrenheit. A supply tank of argon was used to supply the argon at aregulated pressure of 55 pounds per square inch delivered through a A;inch diameter orifice to pressure pipe 38. This resulted in theapplication of a pressure of 18.2 pounds per square inch in the pressureshell 11 and furnace 18. This pressure forced the molten glass throughthe delivery pipe 30 to fill up the space 59 between the molds in 8.1seconds. About 10 pounds of molten glass was delivered to the moldthrough the delivery pipe 30 having an inner diameter of 1% inches andcomposed of a refractory sold under the trade name of Masrock by theGlassrock Corporation of Atlanta, Ga. Thus, approximately 1.2 pounds ofmolten glass per second was delivered to the aperture between the molds.

As a result of filling the space 59 between the molds, the molds and theglass reached a momentary equilibrium temperature of about 1130 degreesFahrenheit at their contacting surface layers within seconds of initialcontact. When the space between the molds was filled, the valve 35 inthe delivery system from the delivery pipe 30 to the space 59 betweenthe molds was rotated from the open position depicted in FIG. 3 to theclosed position depicted in FIG. 4, and crank 73 was operated to causethe jacks 70 to operate in unison to lift the male mold 48 about inch,to increase its vertical distance from the female mold 36. The moldswere then cooled at a slow controlled rate of approximately 30 degreesFahrenheit per hour. Then, the upper mold was separated from the lowermold and the frozen glass hemisphere remaining in the space 49 wasremoved from the lower mold. It had uniform thickness and good opticalsurfaces and conformed accurately to the hemispherical shape desired.

EXAMPLE II Glass hemispheres of the same composition as Example I havinga 56 inch outside diameter and 1% inch thickness are produced by forcingthe molten glass, heated to a temperature of 2680 degrees Fahrenheitbetween an upper male mold and a lower female mold having diameters of53 inches and 56 inches respectively. The molds are maintained atapproximately 900 degrees Fahrenheit before the forced passage of themolten glass into the space between the molds. A delivery rate of 45pounds per second results using a regulated pressure of approximately110 pounds per square inch of nitrogen applied through an orifice of onehalf inch diameter, which results in pressurizing the pressure shell 11at a pressure of approximately 60 pounds per square inch. The moltenglass is supplied to the space between the molds through a delivery pipehaving an inside diameter of six inches. Suitable glass hemispheresweighing 670 pounds having a wall of uniform thickness 1% inches thickare delivered to the space 59 between a male mold weighing 1350 poundsand a female mold weighing 1675 pounds in about 15 seconds. Then, theupper male mold is lifted 1% inches and the mold housing 33 with theinterfacial surfaces of glass and molds at a momentary equilibriumtemperature of between 1100 and 1150 degrees Fahrenheit. Then theseparated complementary male and female molds are cooled at a controlledrate of approximately 15 degrees Fahrenheit per hour to anneal the glasshemisphere and retain its thickness and optical surfaces.

The experiments reported above and other experiments performed toproduce hemispheres of intermediate sizes determined a preferredviscosity for the glass to be less than poises and greater than 10'poises during pressure casting. The male mold should be retracted fromthe female mold when the glass is at a viscosity within the range of 10poises to 10 poises, preferably at approximately 10 poises. A minimumretraction should be a distance suflicient to enable the glasshemisphere, which contracts at a greater rate of thermal contractionthan the graphite molds, to retract away from the upper surface 42 ofthe lower female mold 36 without engaging the lower surface 50 of theupper male mold 48, yet not such a large distance as to lose thebenefits of heat exchange by radiation between the upper mold and theglass. For presently popular hemisphere sizes (one half inch thicknessfor 13 inch outer diameter to 1% inches thickness for 56 inch outerdiameter), the preferred range of separation during annealing rangesfrom 3 to 5 percent of the hemisphere diameter.

The mass of the graphite molds relative to the mass of molten glassintroduced between the molds is an important factor in promoting optimumsurface finish and successful annealing. The thermal conductivity of thegraphite used in the mold determines the relative mass needed. For bestresults, each mold should weigh from 2 to 20 times the weight of glasscast between the molds. Temperature control during annealing is hard tokeep for lower relative mold masses and greater mold masses requiremaintaining mold temperatures approaching the point at which moltenglass adheres to graphite in order to maintain an initial glass coolingrate slow enough to avoid breakage due to stress established duringcooling.

Optimum surface finish is obtained when the surfaces of the molten glassthat contact the mold surfaces at tain a temperature about 50 to degreesFahrenheit above the annealing point of the glass in the short time thatthe main body of molten glass is delivered to the space formed betweenthe graphite molds. This is a surface phenomenon only and does notaffect the temperature of the main body of molten glass immediately. Thetemperature of the main body of molten glass is lowered slowly at ratessufficiently slow to anneal the glass without causing fracture.

The form of the invention shown and described herein represents anillustrative preferred embodiment thereof. It is understood that variouschanges may be made without departing from the spirit of the inventionas defined in the subject matter that follows.

I claim:

1. An apparatus for forming molten glass into a desired shapecomprising:

a pair of upper and lower complementary mold members, each havingcomplementary shaping surfaces spaced from one another to provide a moldcavity therebetween, the lower of said mold members being provided withan opening communicating with said mold cavity,

a pair of heat controlling means for selectively maintaining thetemperature of said mold members at the surfaces of the cavitytherebetween either at (A) a temperature above the annealing point ofthe glass and below the temperature at which the glass adheres to themold members and (B) the temperature at which the glass in the moldcavity is sufficiently viscous to maintain its shape, each of said heatcontrolling means being disposed in heat transfer rela tion to one ofsaid mold members,

a glass melting chamber adapted to contain a body of molten glass, saidchamber being provided with a gas inlet opening,

means for supplying a non-oxidizing gas under pressure through said gasinlet opening to apply pressure to the molten glass in said chamber, and

means for continuously delivering molten glass under the pressureexerted by said non-oxidizing gas from said glass melting chamber andform a location within the body of said molten glass through saidopening in the lowermost of the lower mold member communicating withsaid mold cavity into said mold cavity until said mold cavity issubstantially filled with said molten glass.

2. The apparatus of claim 1 wherein said apparatus includes means forretracting one of said mold members a distance suflicient to permitthermal contraction of the shaped cooling glass in said cavity.

3. Apparatus as in claim 1, wherein said mold members each have a massbetween approximately two times and ten times the mass of a body ofglass completely filling said mold cavity.

4. A mold as in claim 1, wherein said molds are composed of graphite.

References Cited UNITED STATES PATENTS 1,742,098 12/ 1929 Rankin 65304 X3,680,995 8/1972 Frazier 425-243 X 3,420,644 1/ 1969 Lirones et a1.65374 X FOREIGN PATENTS 501,865 3/1939 Great Britain.

ROBERT L. LINDSAY, JR., Primary Examiner US. Cl. X.R.

' UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent -No..3,814,593 Dated June 4, 1974 lnventorh Joseph N. Farris a It iscertified that error appears in the above-idehtified patent and thatsaid Letters Patent are hereby corrected as shown below:

Claim 1, Column- 6, line 44, "form" should be -from-.

Signed and sealed this 1st day of October 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents USCOMM-DC 60376-P69 U.SI GOVERNMENT PRINTING OFFICE I!"0-365-334.

FORM Po-mso (10459)

